Global setting for zero indent added in preamble
\setlength{\parindent}{0pt}
the output::
MWE
\documentclass[11pt]{article}
% \usepackage[demo]{graphicx}
% \usepackage{booktabs}
\usepackage{longtable}
\usepackage{xcolor}
\setlength{\parindent}{0pt}%<______for zero indent in the whole document set to zero
% \setlength{\parindent}{1in}%<_________indent length variation can be set before
%the paragraph or for the whole document
\begin{document}
\pagebreak
\sffamily \large \textbf{test} % this is the problem, here the text is not as left aligned to the left side of the page as it should be; further information: before the \pagebreak is another table
{\setlength{\tabcolsep}{4pt}
\sffamily
\small
\begin{longtable}{p{15cm}}
\caption{tets }\\[2ex]
\hline
\textcolor{black}{test)} \\
\hline
\textcolor{black}{\textbf{test}} \\
\hline
\small test\\
\hline
\textcolor{black}{\textbf{test}} \\
\hline
\small test \\
\hline
\textcolor{black}{\textbf{C}} \\
\hline
\small test \\
\hline
\end{longtable}
}
\end{document}
The indentation can be changed midway in the document by adding
\setlength{\parindent}{1in}
and the output with above 1 inch setting::

PS
Do read the following while using the \parindent command --- https://tex.stackexchange.com/a/55016/197451
EDIT
\fontsize{10pt}{12pt}\selectfont
\begin{verbatim}
% how to set font size here to 10 px ?
\end{verbatim}
The first parameter to fontsize is the font size to switch to and the second is the line spacing to use
testis the start of a paragraph so (depending on code you have not shown) will be indented by paragraph indent. you could use\noindentto suppress that but noindent should never be used, the paragraph indention (or lack of it) should be set up as a global style in the document preamble. – David Carlisle Feb 24 '20 at 23:39